Wearable PCB Design requirements focus on basic Material(1)

                      Wearable PCB Design requirements focus on basic Material (1)

       Because of their size are small, there are few ready-made PCB standards for the growing wearable Internet of things market. And consider how to apply them to unique emerging challenges. There are three areas that require special attention: surface materials for circuit boards, RF / microwave design, and RF transmission lines.

       PCB material

       PCB is generally composed of laminated layers, which may be made of fiber reinforced epoxy resin FR4, polyimide or Rogers or other laminated materials. Insulating materials between different layers are called semi-cured sheets.

      Wearable devices require high reliability, so this will be a problem when PCB designers face the choice of using FR4 (PCB with the best value for money) or more advanced and expensive materials.

       If wearable PCB applications require high speed, the dielectric constant of the high-frequency material. FR4 may not be the best choice. The dielectric constant of the more advanced Rogers 4003 series is 3.55, and that of the brother series Rogers 4350 is 3.66.

FR4 and Rogers 4350.jpg

        The dielectric constant of a stack refers to the ratio of the capacitance or energy between a pair of conductors near the stack to the capacitance or energy between the conductors in a vacuum. At high frequencies, it is desirable to have a small loss, so, Roger 4350 with a dielectric coefficient of 3.66 is more suitable for higher frequency applications than FR4 with a dielectric constant of 4.5.

        Normally, the number of layers of PCB for wearable equipment ranges from 4 to 8. The rule of construction is that, if it is 8, it should provide sufficient strata and power layers and clip the wiring layer in the middle. The ripple effect in crosstalk can be kept to a minimum and the electromagnetic interference can be significantly reduced.

        In the layout stage of the PCB, the layout plan usually depends on the large stratum close to the power distribution layer. This can result in very low ripple effect, and the system noise can also be reduced to almost zero. This is especially important for the RF subsystem.

       Compared with Rogers materials, FR4 has high dissipation factor (Df), especially in the high frequency. For higher performance of the FR4 stack, the Df value is about 0.002. FR4 is better than the ordinary one order. But the Rogers stack is only 0.001 or less. When the FR4 material used in high frequency when applied, will produce significant differences in insertion loss. The insertion loss is defined as the use of FR4, Rogers or other materials when the signal from the power loss of A transmission to B.